The logging while drilling (LWD) technology has been developing rapidly since the 1980s. Compared with conventional wireline logging, LWD provides important information in an oil exploration and development process immediately and accurately, and provides a reliable technical support for improving the operating efficiency. LWD involves acoustics, telecommunications, nuclear magnetism, radioactivity, and other disciplines. In recent years, LWD instruments have been developed. LWD instruments has advanced from monopoles and dipoles to towards multi-pole LWD nowadays. Multi-pole LWD is used for obtaining information such as the compression velocity, the shear velocity, the porosity of a stratum, as well as the permeability and the stability of the borehole wall.
A conventional wireline acoustic logging instrument is affected by drilling noises, drilling fluid circulation noises, and a drill collar wave. A broadband, high-power, and high-efficiency excitation manner is crucial to obtain high-quality acoustic logging data. A traditional acoustic excitation manner commonly adopts a rectangular pulse excitation manner, and a width of a pulse is related to a resonant frequency of a transmitting transducer. In general, the pulse width is one-half of the resonant frequency. However, under such an operating condition, choices for the basic frequency is limited because it requires impedance matching between the power transformer and the transmitting transducer, which strictly operates on the resonant basic frequency of the transmitting transducer. The multi-pole LWD operating at multiple frequency points, if such a method is employed, it is necessary to select a plurality of power transformers and transmitting transducers to achieve resonance at different frequency points. This will greatly increase the design complexity and research and development costs of the instrument.
Nowadays a sinusoidal wave pulse excitation source is commonly employed for the multi-pole acoustic LWD instrument.
Because a rectangular pulse occupies a relatively wide frequency band in a frequency domain, it is relatively low in system power consumption and excitation efficiency in comparison with a sinusoidal pulse excitation. An acoustic LWD transmitting transducer excitation method adopting a sinusoidal pulse is more efficient than a rectangular pulse excitation manner. An excitation emission transducer with peak efficiency may be realized by adopting a sinusoidal wave excitation by changing a frequency of a sinusoidal wave without changing resonance frequency points of the power amplifier and the transmitting transducer.
Currently, three cycles of a sinusoidal wave is commonly used as an excitation pulse of the transmitting transducer. For the multi-pole acoustic LWD instrument, three cycles of the sinusoidal wave require at least two frequency points to excite the transmitting transducer to operate. Monopoles, dipoles, polarizers and quadru-poles do not operate at one frequency point. In the actual operation, the acoustic LWD using the sinusoidal wave excitation has the following two problems:
1. a high-voltage source generated by excitation is always on so that the energy storage capacitor connected to the power source is always in a charged state, which wastes power; and
2. in the case where the power transformer and the transmitting transducer do not operate at a resonant frequency point (in general, multi-pole acoustic excitation operates at two or more frequency points), or are not matched well. In a three-cycle sinusoidal wave excitation process, a magnet inside the power transformer stores energy. Once the three-cycle sinusoidal wave excitation is over, the power transformer will release the stored energy, creating a shock trailing smear (so-called high-voltage ringing effect), behind a three-cycle sinusoidal wave excitation signal, which has a negative impact on an excitation effect of the transmitting transducer.
These two problems will increase the system power consumption, and reduce the excitation efficiency of the transducer.